The recent emergence of barrier infrared detectors such as the nBn and the XBn have resulted in mid-wave infrared (MWIR) detectors with substantially higher operating temperatures than previously available in III-V semiconductor based MWIR detectors. The initial nBn devices used either InAs absorber grown on InAs substrate, or lattice-matched InAsSb alloy grown on GaSb substrate, resulting in cutoff wavelengths of ˜3.2 micron and ˜4 micron, respectively. While these detectors could operate at much higher temperatures than existing MWIR detectors based on InSb, their spectral range does not cover the full MWIR atmospheric transmission window (3-5 micron). There have also been nBn detectors based on the InAs/GaSb type-II superlattice absorber, and, although these InAs/GaSb superlattice based detectors have sufficiently long cutoff wavelength to cover the entire MWIR atmospheric transmission window, they have not achieved very high performance levels.
While digital alloys based on periodic insertions of thin layers of InSb into InAs or InAsSb hosts have been proposed (and demonstrated) as absorbers for barrier infrared detectors, these types of digital alloy based barrier infrared detector have exhibited extended cutoff wavelengths, they still have limited spectral range. The InAs/InAsSb superlattice has been used in midwave infrared (MWIR) lasers. (See, Yong-Hang Zhang, Appl. Phys. Lett. 66(2), 118-120 (1995); and A. Wilk, M. et al., Appl. Phys. Lett. 77(15), 2298-2300 (2000), the disclosures of each of which are incorporated herein by reference.) In addition, it has been suggested that the InAs/InAsSb superlattice may be suitable for longwave infrared (LWIR) detector applications. (See, Yong-Hang Zhang, pp. 461-500, in Antimonide-Related Strained-Layer Heterostructures, edited by M. O. Manasreh, Gordon and Breach Science Publishers, Amsterdam (1997), the disclosure of which is incorporated herein by reference.) Recent papers on growths of strain-balanced InAs/InAsSb superlattices on GaSb substrates also suggest that they may be useful for infrared detector applications, as they demonstrated photoluminescence or photoconductive response in the infrared. (See, D. Lackner, et al., Appl. Phys. Lett. 95, 081906 (2009); D. Lackner, et al., “InAsSb and InPSb materials for mid infrared photodetectors,” 2010 International Conference on Indium Phosphide & Related Materials (IPRM) (2010); and Y. Huang, J.-H. Ryou, et al., J. Crystal Growth 314, 92-96 (2011), the disclosure of each of which are incorporated herein by reference.) However, even with a high quality infrared absorber material as the starting point, building a high-performance infrared photodetector still demands considerable sophistication, as it requires the intricate interplay among many building components.
Infrared detector performance depends strongly on device design. The use of heterostructure designs to enhance infrared detector performance is a well-established practice, and is prevalent in III-V semiconductor based infrared detectors. A particularly useful heterostructure construct is the unipolar barrier, which can block one carrier type (electron or hole) but allows the substantially un-impeded flow of the other, as illustrated in FIG. 1. (See, D. Z.-Y. Ting, et al., Appl. Phys. Lett. 95, 023508 (2009), the disclosure of which is incorporated herein by reference.) Unipolar barriers have also been used extensively to enhance infrared detector performance. White used unipolar barriers to block the flow of majority carrier dark current in photoconductors without impeding minority carriers. (See, U.S. Pat. No. 4,679,063, the disclosure of which is incorporated herein by reference.) A double heterostructure (DH) detector design can be used to reduce diffusion dark current emanating from the diffusion wings surrounding the absorber layer. (See, M. Carras, et al., Appl. Phys. Lett. 87(10) 102103 (2005), the disclosure of which is incorporated herein by reference.) The nBn or XBn detector structure uses a unipolar barrier to suppress dark current associated with Shockley-Read-Hall processes without impeding photocurrent flow, as well as to suppress surface leakage current. (See, e.g., S. Maimon and G. W. Wicks, Abstract Book of the 11th International Conference on Narrow Gap Semiconductors, Buffalo, N.Y., p. 70 (2003); S. Maimon and G. W. Wicks, Appl. Phys. Lett. 89(15), 151109 (2006); U.S. Pat. No. 7,687,871 B2; WO 2005/004243 A1; and P. C. Klipstein, Proc. SPIE 6940, 69402U (2008), the disclosures of each of which are incorporated herein by reference. Other conventional detector devices can be found in the following references, the disclosures of each of which are incorporated herein by reference: WO 2008/061141; U.S. Pat. No. 7,795,640; U.S. Pat. No. 4,679,063; US Pub. No. 2007/0215900; US Pub. No. 2010/0006822; US Pub. No. 2009/0127462; and U.S. Pub. No. 2010/0155777.)
In general, unipolar barriers can be used to implement the barrier infrared detector architecture for increasing the collection efficiency of photo-generated carriers (by deflecting them towards the collector, in the same way a back-surface field layer functions in a solar cell structure), and reducing dark current generation without inhibiting photocurrent flow. However, despite the substantial advantages they offer, unipolar barriers are not always readily attainable for the desired infrared absorber material, as the proper band offsets must exist between the absorber and the barrier, and both the absorber and barrier materials require (near) lattice matching to available substrates on which they are grown.
Another construct that is useful in building high-performance heterostructure infrared detectors is material with graded band gap. Graded-gap (or chirped) material is useful in creating a quasi-electric field for driving carriers in the desired direction, and for smoothly connecting two regions with different band gaps.
Accordingly, a need exists to develop barrier infrared detectors that incorporate the properties of the these novel superlattice absorbers, as well as their matching unipolar barriers and graded gap materials to form detectors capable of operating in 3-12 micron spectral range covering 3-5 micron and/or 8-12 micron atmospheric transmission window, at high temperature, and with high performance.